1. Technical Field of the Invention
The invention concerns a method for determining the flow rate and/or the molecular mass of liquid or gaseous media in flow measuring instruments, especially pneumotachographs, with the aid of the measurement of the transit time of ultrasonic pulses in the medium between two ultrasonic transmitters/receivers, especially piezoelectric transmitters/receivers, as well as a flow measuring instrument for executing the method.
2. Description of the Prior Art
The determination of the flow rate and molecular mass of gaseous media is of interested in industry as well as in medicine, e.g. when recording various lung function parameters such as tidal volume or gas exchange in the lung. To that end, measuring instruments are known, which function with different physical principles. In the case of ultrasonic transit time methods, the change in the speed of propagation of sound waves in relation to the moving medium is used in order to determine of the flow rate and/or the molecular mass. An ultrasonic pulse is transmitted essentially in the direction of movement through the medium whose transit time is measured over the defined distance between the transmitter and the receiver, and the procedure is repeated immediately thereafter in the opposite direction. Thereby, the speed of propagation of the pulse through the flow of the medium is increased or lowered, and from the different values, the flow rate or the molecular mass can be determined. The method can also be applied to liquid media. The physical principles and the formulae for calculating the desired values can be taken from the articles xe2x80x98Technologie der Ultraschall-Spirometriexe2x80x99 by C. Buess, W. Guggenbxc3xchl and K. Hamoncourt in the journal xe2x80x98Atemwegsund Lungenkrankenheitenxe2x80x99, 21st year, no. September 1995, pp 438-442, and xe2x80x98Die Molmasse der Atemluftxe2x80x99 by T. Kenner and K. Harnoncourt, ibid, pp 454-456, with the bibliographical references specified there. In medicine engineering, pneumotachographs, which use this measuring principle, are known and in use, whereby either the time between emittance and reception are measured directly electronically, i.e. counted by a clocked counter, or if a transmitter transmits ultrasonic waves continuously, the phase displacement is determined in the receiver. The required measures are known to a person skilled in the art. Standard pneumotachographs have flow pipes with a diameter of 2-3 cm so that the transmitters/receiver have a mutual spacing of approx. 5 cm, and this returns transit times of some 150 microseconds. The flow rate that occurs during normal breathing of 1 m/s results in a change of the transit time in the magnitude of one microsecond, as can be calculated using the above mentioned formulae. Moreover, the composition of expired air, which contains approx. 5% CO2 by volume compared with the inspired air, changes the transit time in this magnitude. As ultrasonic transmitters/receivers, preferably piezoelectric quartz resonators are used with a resonance frequency of several 100 kHz, as can be taken from the state-of-the-art. They offer the advantage of high frequency stability, and a short settling time so that the temporal interval between the reception and transmission of a pulse by a quartz resonator is very short, and during this period the flow conditions in the flow pipe hardly change and corrupt the measuring result. With the simultaneous emittance of pulses from both transmitters, the measurements would be corrupted through overlay of reflected sound waves.
A disadvantage of the methods known until now for determining the transit time consists in the type of transit time measurement. As described above, the time changes only by a few microseconds so that the actual measurement is only relevant in a short time segment around the value of the transit time in the standing medium. In order to resolve one microsecond with sufficient accuracy, it should be divided at least into 1.000 time intervals, that is, effect a measurement in the nanosecond range. The clocked counters used to count the time between transmission and reception of the signal function with frequencies in the GHz range. Because, however, the total transit time is counted, a relative error of, for example, only 1% results already in corruptions that lie in the magnitude of the transit time change. A more accurate time resolution, i.e. a faster counter, is moreover hardly realisable.
The invention has the object of measuring the transit time of an ultrasonic pulse in a moved medium with sufficient accuracy.
This task is solved according to the invention therein that an ultrasonic pulse is emitted from a transmitter, and simultaneously the run of a time duration starts, which is slightly shorter than the transit time in the static medium, after the end of the time duration the discharge of a capacitor, to which a voltage is applied, commences with a constant intensity of current, the discharge is terminated with the registration of the ultrasonic pulse in the receiver, the voltage still applied to the capacitor is measured and the discharge time is determined, the transit time from the time duration and the discharge is added up, the measurement of the transit time is repeated in the opposite direction, the flow rate and/or the molecular mass is determined from the different transit times.
The central idea of the invention relates to the concept that the actual time measurement commences only briefly before the probable arrival of the ultrasonic pulse in the receiver, and the charge stored in a capacitor is used for the time measurement. As explained above, the transit time is subject to only minor changes so that the actual measurement does not have to be started until briefly before the end of the known transit time in the static medium. It can be determined with a dry run, i.e. without flow, and then a slightly shorter time duration be set, e.g. 148 instead of 150 microseconds with a transit time change of approx. 1 microsecond, as the so-called dead time, upon expiry of which the discharge of a capacitor is commenced. Through application of a voltage, the capacitor is charged with a known charge. It is possible for the person skilled in the art to discharge the current with a constant intensity so that the charge stored in the capacitor and thus the voltage is reduced continuously. With the registration of the pulse in the receiver, the discharge process is then terminated. The voltage applied to the capacitor is converted into the discharge duration of the capacitor, according to the formula t=(Cxc3x97U): I, where the capacitor capacity is C, the voltage U and the constant discharge current I. It is added to the dead time and the total transit time of the ultrasonic pulse from the transmitter to the receiver obtained. After the process has been repeated immediately subsequently in the opposite direction, i.e. the previous receiver acts as a transmitter and vice versa, the transit times of the ultrasonic pulse are measured with the direction of movement and against the direction of movement, and with the known formulae the flow rate and the molecular mass of the medium can be calculated. The measures required, i.e. the coupling of a receiver to a capacitor, and the determination of the discharge time and the addition to the dead time and the calculation of the desired measuring result, can be effected automatically through electronic circuits, as is known for the trained person skilled in the art.
The advantage of the invention consists therein that the measurement of electrical quantities, such as the voltage applied to the capacitor in mV, can be executed with cost-effective measuring instruments and insignificant measuring errors. Furthermore, the analog output signals can be processed with known electric circuits, e.g. operational amplifiers, so that the evaluation of the measuring results, i.e. the summation and subtraction of the transit times, is realisable in a simple way. Moreover, by means of the shorter time duration, during which the measurement is executed, there is a reduction in the absolute magnitude of the relative measuring error, which occurs with every measurement, because an error of 1% in the case of a microsecond corrupts the result less as in the case of the measuring methods described above. This method can be used in the broadest range of fields, also industrial and commercial. For instance, it is possible to measure the intake rate of air in a combustion engine, or the flow of fuel in a line.
Advantageous embodiments of the method and an instrument to execute the method are the object of subclaims.
To adapt the method to the broadest range of gaseous or liquid media, an advantageous embodiment consists therein to change the time duration and/or the voltage applied to the capacitor and/or the discharge voltage. Because in different media the ultrasonic pulses have different sound velocities, the time duration which expires prior to the commencement of the measurement is adjusted. With a change in the voltage applied to the capacitor and in the discharge current, an adjustment to the required measuring range is achieved. If, for example, the capacitor current is increased, with a constant discharge current, a longer time duration can be measured, as is necessary with a faster flow and a related large change in the transit time. Likewise, it is possible simultaneously to increase the discharge current so that the reduction, e.g. by one mV, in the capacitor current is always assigned to the same time period in order to obtain a constantly high resolution of the respective measuring range.
A simplification of the method consists therein to determine the transit time of a pulse in the static medium, and to adjust the voltage applied to the capacitor and/or the discharge current so that the voltage 0V, which is reached after a particular discharge duration, is assigned to the transit time in the static medium, i.e. that the zero crossing of the capacitor current, when a positive voltage has been applied, corresponds to the instant at which a pulse in the static medium would have been registered in the receiver. Thereby the measuring range, i.e. the voltage and the discharge current intensity, are adjusted to the respective medium with its flow rate. The advantage consists therein that with a linear approach, as described further below, the small changes in the transit time, or the measured applied voltage, upon termination of the discharge, are directly convertible with the aid of known circuits into the change of the flow rate or the molecular mass, whereby the transit times each deviate by the same small value from the time in the static medium.
In a simplification of the calculation of the flow rate and/or the molecular mass of the medium, it is assumed that a small change of the flow rate is proportional to a small change in the transit time, or conversely proportional to the change in the molecular mass. With the formula for calculating the flow rate, this linearisation is presented exemplarily: it contains the term (t1xe2x88x92t2)/(t1*t2), with the two different transit times which each differ by the small amount d from transit time t0 in the static medium. This yields: ((t0+d)xe2x88x92(t0xe2x88x92d))/(t02xe2x88x92d2)=(2*d)/t02. Because d2 is small in the second order, it is negligible how an estimation of the magnitudes obtains: t0 in the sample embodiment of the drawing is 150 microseconds and d approximately one microsecond so that there is a relative error of 0.005%. It follows that the small change in the flow rate is proportional to the small transit time change d. This approximation of course applies to only a limited range, which is to be determined for each medium and each flow rate, i.e. so long the deviation from the actual value is smaller than the measuring error of the device. The advantage of linearisation consists therein that the voltages measured at the capacitor in mV can be converted directly into the change of the flow rate or the molecular mass, which can be executed easily with the aid of known circuits.
A possible method for determining the molecular mass concerns therein to effect the determination in the static medium. This has the consequence that the transit times of the ultrasonic pulses, which are transmitted in the opposite direction through the medium, have the same amount. Here it is irrelevant according to the invention how the medium is brought to a quiescent condition in the flow pipe in relation to the pipe.
The apparative design of a flow measuring instrument, which is based on the novel measuring method, is similar to that of a known flow measuring instrument, as used in medicine or measurement techniques in the broadest range of the fields. The ultrasonic transmitters/receivers here are piezoelectric quartz resonators, which are directly coupled to a capacitor to determine the discharge time. The implementation of corresponding control electronics, which start the discharge process and stop it upon reception of the ultrasonic pulse and according to one of the methods described above determine the transit time, is possible for a person skilled in the art. The advantage with such an instrument consists therein that the new method can be performed with existing instruments, and only the control and evaluation electronics on the quartz resonators are modified, whereby the signals are available in analog form and can be converted in a known way into the flow rate or the molecular mass, e.g. with the aid of operational amplifiers.